New ULF wave index as indicator of turbulent level of the magnetosphere and IMF

1. New ULF wave index as indicator of turbulent level of the magnetosphere and IMF V. Pilipenko, O. Kozyreva Institute of the Physics of the Earth, Moscow J. Watermann, O. Rasmussen Danish Meteorological Institute, Copenhagen Yumoto, K. Kyushu University, Fukuoka Engebretson, M.J. Augsburg College, Minneapolis, MN

2. Necessity of new indices The interaction between the solar wind and magnetosphere is the primary driver of many processes in the near-Earth space environment. This interaction has often been viewed using the implicit assumption of quasi-steady and laminar plasma flow. However, the energy transfer processes in the magnetospheric boundary regions have a turbulent character. Progress in understanding and monitoring time-varying processes in space physics is hampered by the lack of convenient tools for their characterization. Various geomagnetic indices (Kp, Dst, AE, PC, epsilon, etc.) quantify the energy supply in certain regions of the solar wind-magnetosphere-ionosphere system. However, these indices characterize the steady-state level of the electrodynamics of the near-Earth environment. The turbulent character of solar wind drivers and the existence of natural MHD waveguides and resonators in the ULF frequency range (~1-10 mHz) ensures a quasi-periodic response to forcing at the boundary layers. Therefore, much of the turbulent nature of solar wind-magnetosphere-ionosphere interactions can be monitored with ground-based ULF observations. So far, there is no index characterizing the turbulent character of the energy transfer from the solar wind into the upper atmosphere and the short-scale variability of near-Earth electromagnetic processes. There are many space weather related problems, where even a rough proxy of the level and character of low-frequency turbulence, which might be coined a “ULF wave index,” is of key importance.

3. Solar wind - magnetosphere as a turbulent high-Reynolds-number system • Though the solar wind (SW) is turbulent, in studies of SW-magnetosphere coupling this fact is ignored, and only time-averaged IMF and SW parameters are used. • However, a laminar flow is not an average of a turbulent flow! SW may drive the magnetosphere in a different manner, depending on the upstream turbulence level. • The eddy (turbulent) viscosity of the SW flow passing the magnetosphere is controlled by the level of upstream turbulence, and that the degree of coupling of the SW flow to the magnetosphere is controlled by the level of IMF/SW turbulence. • From laboratory fluid dynamics it is known that the coupling of a fluid to an obstacle is determined by dimensionless Reynolds number Re=VL/ν(stream velocity V, the molecular viscosity ν, the obstacle size L).Re is to be constructed with the turbulent viscosity rather than the kinematic viscosity! In SW/magnetosphere coupling, the level of turbulence in the magnetosheath determining an eddy viscosity may control the degree of driving of the magnetosphere. • The presence of turbulence inside and outside the magnetosphere should have profound effects on the large-scale dynamics of the system through turbulent viscosity and diffusion. For example, there is a specific class of magnetospheric disturbances – Sawtooth events, i.e. quasi-periodic global magnetospheric response with time scale ~2 hours to the SW driving under steady IMF Bz<0. Recent studies indicated that these events are best discriminated from other similar events, such as quasi-periodic substorms or steady magnetospheric convection (SMC) by the IMF turbulence level! Therefore, an interplanetary ULF wave index could be of great help in studies of these events.

4. Impact of the Level of Solar-Wind Turbulence on Auroral Activity A naive expectation is that when the SW is more turbulent, the effective degree of its coupling to the magnetosphere is higher. Auroral response is compared with with similar strength of the SW driver (Bz) for the laminar and turbulent wind flow: • IMF is noisy (var{Bz}>2nT); • IMF is calm (var{Bz}<2nT). The average AE values for the turbulent SW are higher than for the laminar solar wind! This difference is most significant for northward Bz, when one expects the viscous interaction to be dominant over the reconnection. This comparison reveals that the magnetosphere is driven more weakly when the level of SW turbulence is low. In studies of SW-magnetosphere coupling the SW turbulence is ignored, and ULF turbulence index for the SW must be introduced.

5. “Killer” electrons and satellite anomalies Decline of solar activity, no SPE, but satellite anomalies have not disappeared!? The main menace - from the relativistic electrons! Increases of electrons E=1.8-3.5 MeV detected by LANL produce swarms of malfunctions ULF wave activity – a driver of relativistic electrons?! To validate this mechanism an index characterizing monochromatic ULF activity in the magnetosphere is necessary

6. Geosynchrotron: ULF waves = intermediary between the solar windand “killer” electrons!? Appearance at GEO of relativistic electrons following storms resists definitive explanation. These electron events are not merely a curiosity for scientists, but they can have disruptive consequences for spacecrafts. While it has been known a general association between storms and electron enhancements, the wide variability of the response and the puzzling time delay between storm main phase and the peak of the response has frustrated the identification of responsible mechanisms. Some intermediary must more directly provide energy to the electrons?! Rather surprisingly, ULF waves in the Pc5 band (1-10 mHz) have emerged as a possible energy reservoir: the presence of Pc5 wave power after minimum Dst is a good indicator of relativistic electron response [O’Brien et al., 2001]. In a laminar, non-turbulent magnetosphere the “killer” electrons would not appear! Mechanism of the acceleration of ~100 keV electrons supplied by substorms is revival of the idea of the magnetospheric geosynchrotron. Pumping of energy into seed electrons is provided by large-scale MHD waves in a resonant way, when the wave period matches the multiple of the electron drift period

7. Ring current dynamics There is a view that RC development results from a sustained enhancement of the convection E driven by the IMF/SW. In this view it is implicitly assumed that there must be some secondary, relatively efficient and continuous, process that scatters particles from open to closed drift paths: fluctuations in the SW(?), ULF waves in the magnetosphere(?).This process, though being of key importance, is not observable in any existing indices. Wave precursors of substorms The variability of SW and magnetospheric conditions might be an important factor in triggering magnetospheric substorms, but this idea has not been thoroughly examined so far. Enhanced reconnection and viscous interaction in dayside boundary regions, leading eventually to substorms, are to be accompanied by an enhanced level of turbulence. Therefore, substorm break-up may be preceded by an increased level of ULF power in the dayside cusp. There are events indicating the occurrence of the broadband ULF variations in the nominal dayside cusp region before substorm break-up and sudden suppression of ULF activity after it. Pre-heating of the plasmasheet plasma owing to the resonant absorption of MHD turbulence may provide necessary conditions for the onset of an explosive instability, resulting in a substorm break-up (“thermal catastrophe” by Goertz and Smith [1989]). Application of statistical methods for the search for wave precursors of substorms will benefit from the development of an index quantifying ULF activity.

8. Example of possible dayside wave “precursor” of nighttime substorm

9. ULF noise in seismo-active regions Anomalous ULF noise ~ 0.01 Hz may occur a few days before strong earthquakes, caused by the crust micro-fracturing at the final stage of the seismic process. Validation of this effect on a large statistical basis with the use of magnetic stations in seismo-active regions will be possible only with the use of a proper ULF wave index. This index will provide the seismic community with an effective tool to distinguish local e/m ULF wave anomalies from global enhancements

10. Construction of the ULF wave index • The wave index as a proxy of global ULF activity is reconstructed from the one-min data from the arrays of magnetic stations in Northern hemisphere: • INTERMAGNET • Greenland Coastal Array+MAGIC • MACCS • CPMN (210 Magnetic Meridian Chain) • Russian Arctic magnetic stations • DMI WDC selected observatories

11. Algorithm of the ULF wave index • Data are detrended (cut-off frequency 0.5 mHz) and converted into X,Y coordinate system. • For any UT, magnetic stations in the MLT sector 05 – 15, and in the latitudinal range 65° - 75° CGM are selected (to avoid substorm-related disturbances at the night hours). • For data visualization the components H,D,Z are plotted for each selected station (with noon and midnight marks). • The dynamic spectra of two horizontal components are calculated with the use of Filon’s method in 1 hour time window. • The most promising frequency range for the index definition is the Pc5 band (~ 2-7 mHz) with the most intense fluctuations (Nayquist frequency is 8.3 mHz). • Global ULF wave index is calculated as follows The summation is performed with respect to all N stations where the signal amplitude is above K*Bmax (parameter K is between 0.5 and 1.0).

12. Broad-band and narrow-band ULF waves • The usage of a wave index based on band-integrated wave power only may be misleading, because this type of index cannot discriminate between irregular wide-band variations and narrow-band waves! It is necessary to apply to the analysis of ULF dynamics a measure R of the fraction of narrow-band pulsations in observed wave power. For the discrimination of broad-band and narrow-band variations the following algorithms are used: • the ratio between the wave power in a narrow band (2-10 mHz) and wide band (0.2-10 mHz); • the bump above the log-log fit to a spectrum. 2-nd method: Spectral functions are transformed into the log-scale. In this scale the “colored-noise” spectrum is a straight line The log FN (f) is approximated by linear fit by minimizing the chi-square error statistics in the band 1-8 mHz.

13. Discrimination of noise and signal from ULF spectra The total power sub-index should be augmented by a “signal” sub-index to discriminate between broad-band and narrow-band ULF waves. Noise band-integrated power is the area beneath the background spectra Signal spectral power is the area of the bump above the background spectra

14. Illustration: Space Weather Month (September 1999) Two geoeffective solar wind intervals: 22-23 September, an interval with a strong magnetic storm (P ~15 nPa, and Dst = -143 nT) caused by magnetic cloud; 26-29 September, an interval with high solar wind streams (V ~ 650 km/s) and series of substorms. The first s/s during magnetic storm on September 22 occurred on ~20 UT, another happened on September 23, at ~04 UT. These two s/s were accompanied by short-lived and low-intensity Pc5 waves. On contrary, a series of s/s started on September 26 ~16 UT, September 27, at ~06 UT, and ~14 UT all were succeeded by very intense and long-lasting monochromatic Pc5 activity. Analysis of relativistic electron observations at geostationary satellite has shown, perhaps unexpectedly, that a significant increase of relativistic electron flux at geostationary orbit (up to 2-3 orders of magnitude!) was observed not during the magnetic storm, but during the second interval with elevated ULF wave activity.

15. Final product: the zoo of hourly ULF wave indices • A similar ULF GEO wave index is calculated from 1-min 3-component magnetic data from GOES satellites to quantify the short-term magnetic variability in the region of geostationary orbit. This value would be especially helpful for the problem of ULF energy pumping into the relativistic electrons. • To quantify the short-term solar wind and IMF variability, the interplanetary ULF wave index is calculated from the 1-min IMF data from the interplanetary satellites IMP8, WIND, ACE. The data are time-shifted to the terrestrial bow shock (15 RE). • The basic space weather parameters are taken from the OMNI-2 database. • The final output monthly files have the hourly values of the following parameters: • SW velocity & density, IMF components • standard static indices (Dst, AE) • ground global ULF wave indices (T, N, S) • GEO ULF indices (T, N, S) • IMF variability indices (T, N, S) • A wide range of space physics studies, such as substorm physics, relativistic electron energization, SW-magnetosphere-ionosphere coupling, etc. will benefit from the introduction of the ULF wave index.

16. SW/IMF driving of the magnetosphere and “killer” electron response during Space Weather Month

17. Magnetic vector B variations during the September 1999 solar wind streams Example: March 1998 storm

18. Global ULF wave indices during Space Weather Month

19. Possible interpretation of the wide variability of the electron response to magnetic storms and the puzzling time delay between storm main phase and the peak of the response ULF activity during the main phase of the magnetic storm is basically broad-band and short-lived. Oscillations are caused by other mechanisms than typical Pc5, and their features (spectrum, transverse spatial scale) do not match the conditions necessary for the electron resonant acceleration by ULF waves. ULF activity in the recovery phase is narrow-band and long-lasting. These pulsations might be related to the gradual increase of relativistic electron fluxes owing to drift-resonance acceleration. Mechanism of the acceleration of ~100 keV electrons supplied by substorms is revival of the idea of the magnetospheric geosynchrotron. Pumping of energy into seed electrons is provided by large-scale MHD waves in a resonant way, when the wave period matches the multiple of the electron drift period.

20. Scientific consortium comprising Space Reserch Institute (Moscow), Space Center of Augsburg College (Minneapolis), Space Environment Research Center of Kyushu University (Fukuoka), Institute of the Physics of the Earth (Moscow), and Danish Meteorological Institute (Copenhagen) will provide the space community with a new ULF wave index, analogous to geomagnetic indices, derived from ground-based and satellite observations in the ULF frequency band – the range of natural MHD resonators and waveguides. The turbulent level of the SW-magnetosphere-ionosphere system can be monitored with this index. The database for interval 1997-2001 is freely available to space community via mirror anonymous FTP site for testing and validation: space.augsburg.edu folder:/pub/MACCS/ULF_Index/ Mirror FTP sites will be created soon in the DMI and Kyushu University CD with ULF index database may be requested! Comments, suggestions, and requests are welcomed: Pilipenk@augsburg.edu Kozyreva@ifz.ru Engebret@augsburg.edu Yumoto@geo.kyushu-u.ac.jp jfw@dmi.dk